A convective treatment system consists of a temperature-control/blower unit (known simply as a “blower”), a ducting system, a convective device such as a convective warming blanket, and/or an infusate heat exchanger. A blower aspirates air from an ambient environment, changes its temperature to a desired value, pressurizes the air above the ambient pressure, and discharges the air at an exhaust port. U.S. Pat. No. 6,126,393 describes such a blower and associated temperature and noise control schemes. In an exemplary convective treatment system, pressurized, thermally regulated air produced by a blower is conveyed through a ducting system and delivered to a convective device, such as a convective warming blanket, that distributes the thermally regulated air around a person or a specific body area of a person. A person can be a human being, animal, or thing. In some applications, a blower unit may be used to operate other accessory devices with or without an convective device. In these stand-alone applications, the blower unit may be used to warm infusates, such as blood or saline, through the use of a heat exchanger adapted to fit within the duct system. U.S. Pat. No. 5,807,332 describes one type of nonconvective device that is used to warm infusates for administration into persons. The use of an infusate warmer does not preclude the concomitant use of a convective device; however, the distal end of the air supply duct is covered with an air diffuser during the exclusive use of an infusate warmer. The user must intentionally place the diffuser over the distal end of the air supply duct. The diffuser allows the heated air to escape from the distal end of the air supply duct but prevents the heated air from striking the patient directly.
A convective device may be embodied, for example, in an inflatable device which inflates with pressurized, thermally regulated air and has one or more surfaces adapted for expelling air onto a person. Such devices may lie on, around, or under the person. A convective device is generally realized as a blanket, but can be embodied by other appliances or attachments that are designed to be operated by or with the application of pressurized, thermally conditioned air. When used herein, the term “convective device” is intended to include all blankets, pads, covers, manifolds, and equivalent structures that operate as just described. Irrespective of orientation, a convective device utilized for convective thermal treatment of persons performs at least three basic functions. These functions are 1) the conveyance of thermally conditioned air from at least one inlet port into the device, 2) the imposition of a heat gain or loss that changes the temperature of the thermally conditioned air, and 3) the extravasation of the thermally conditioned air from the device. In the following discussion, the assumption is that such a convective treatment device is operated to warm a person by delivery of heat to the person.
In those convective treatment systems which warm a person by the application of heat, heat may be transferred by convection, radiation, and conduction, but convection generally predominates at the interface between the convective device and the person. The rate of convective heat transfer depends on material properties, surface boundary conditions, and significantly, fluid velocity.
Heat is lost from a convective treatment system whenever a temperature gradient exists between it and the ambient environment. During normal operation of the system, the temperature of the air expelled onto the person is maintained at a level that is generally higher than the person's skin surface temperature, but not high enough to cause tissue damage. In order to counter the loss of heat from the system, however, the air is heated initially to a temperature that may exceed the thermal damage threshold at the target site on the person's skin. Within certain limits, the amount of heat lost from the system is predictable. This predictability allows the system to operate safely by measuring and controlling the temperature at the proximal end of the air supply duct that connects the blower to the convective device. If any factors upon which the assumption of predictability depends are altered, however, the fluid temperature at the distal end of the duct system may be affected.
Several intrinsic and extrinsic factors contribute to the rate of heat loss from a convective treatment system. Among the intrinsic factors are the surface area and material characteristics of the duct and convective device, and the residence time of the warmed air within the duct and convective device. Extrinsic factors include, but are not limited to, ambient temperature and air velocity in the area immediately adjacent to the duct and the convective device. The residence time of the heated fluid within the system is a function of its pressure and the resistance exerted by the entire system. Factors that influence resistance are the duct diameter and length, the orientation of the duct, and the resistance of the convective device or devices.
One hazard associated with the use of convective treatment is burns. First-, second-, and third-degree bums have occurred through the improper use of convective treatment systems. The bum hazard is accentuated by the intentional or accidental alteration of the intrinsic or extrinsic factors that moderate the heat loss in the system. The alteration of any of these factors introduces an unpredictable amount of heat loss into the system, which can significantly alter the temperature or velocity of the heated air delivered to the person. One of the more important factors that influence the temperature of warm air flowing out of the air supply duct through the end where it connects to the convective device is the residence time of the air within the duct. The end through which air flows out of the air supply duct is usually referred to as the “distal end” of the air supply duct. Typically, a nozzle may be mounted to this end. The temperature of pressurized warm air exiting the duct at this end is called “nozzle temperature” (whether or not a nozzle is mounted thereto). In general, a decrease in residence time of the pressurized warmed air is usually associated with an increase in the nozzle temperature of the air.
In the field, a common misuse of one or more components of a convective treatment system occurs. Either intentionally or accidentally, some users fail to connect the convective device to the distal end of the duct and allow the heated air discharged from the distal end to make direct contact with the person. In view of the fact that an air supply duct is typically embodied as an air hose, this practice has come to be known as “hosing” or “free-hosing.” In other cases, operators have failed to connect the convective device to the duct and allowed the heated duct to make direct contact with the person's skin. Users who have experienced therapeutic misadventures through this type of misuse have reported their experiences of thermal injuries to the FDA and the manufacturers of the offending convective treatment systems. Some manufacturers of have responded by warning and training users and affixing labels to the thermal-control/blower units and convective devices. Despite warnings, training, and labeling, however, persons continue to be injured through misuse of warning devices.
The American Society for Testing and Materials (ASTM) has recently circulated a draft standard (ASTM F29.19.01) from the Subcommittee for Patient Warming Equipment entitled Standard Specification for Circulating Liquid and Forced Air Patient Temperature Management Devices. The members of the ASTM subcommittee recognized the hazards associated with the practice of free-hosing and developed requirements for equipment to limit skin surface temperatures to 48° C., or less, during any operating or fault condition. Additionally, the standard requires the manufacturers of thermal-control/blower units to affix a cautionary statement to the distal end of the air supply duct that warns the user against the practice of “free-hosing.” Thus, the ASTM standard explicitly recognizes the importance of air temperature, and tacitly acknowledges the role of airflow, in causing thermal burns.
Hosing causes at least four uniquely hazardous conditions to exist: 1) The loss of the resistance from the lack of an convective device leads to a decrease in the residence time of warmed air in the air supply duct. As the warmed air has less time to cool in the air supply duct, it arrives at the distal end of the duct at a higher than normal temperature; 2) The lack of airflow resistance from the absence of the convective device also leads to an increase in the air velocity that is exhausted from the supply duct; The relative increase in air velocity can lead to significantly higher heat transfer rates if the air strikes the skin; 3) The lack of an convective device makes it possible for the high temperature and high velocity air to strike directly the person's skin over a very small area. In essence, all, or most, of the heat energy intended to be distributed over a large surface area is concentrated onto a very small area; and 4) The lack of an convective device makes it possible for the air supply duct itself to make direct contact with the person's skin.
It is manifest that the hazards of hosing are not intentionally visited on any victim. Nevertheless, it is the case that large caseloads and near-crisis conditions can distract the attention of those who are in charge of the immediate operation of convective treatment systems. In such circumstances, the practitioner may be unaware of the development of conditions that pose a hazard of bums, or may be forgetful of known conditions that require close and constant attention. Accordingly, significant benefits would be realized by safety provisions that operate automatically to reduce the risk of harm that can arise during the operation of convective treatment systems. Especially desirable are measures that would automatically mitigate the potential of burns that might occur when the air supply duct is separated from the convective device in a convective treatment system that delivers warmed air for treatment.
The assignee of this application has designed safety provisions that reduce the risk of burns by modulating the operation of a blower in response to changes in the integrity of the connection between the air duct and the convective device. These provisions are set out in U.S. Pat. No. 6,128,681 and a continuation-In-part thereof, U.S. patent application Ser. No. 09/546,078, now U.S. Pat. No. 6,447,538both of which are Incorporated herein by this reference. However, these provisions must be implemented in the structure and operation of a blower, and Implicate redesign and reconstruction of exiting blower architecture
Accordingly, there is an immediate need for additional, easily-implemented measures in convective treatment technology to 1) automatically mitigate a potentially unsafe condition irrespective of an operator's awareness of the unsafe condition, 2) prevent the intentional or unintentional misuse of convective treatment system components by users who fail to connect the appropriate convective devices to the distal end of the air supply duct and thereby allow heated air to make direct contact to the person, and 3) prevent the air supply duct from causing thermal injury to the person if it makes direct contact with the person's skin.